This invention relates to a meter for determining the internal impedance of an individual battery cell within a battery backup system without disconnecting the battery cell from the backup system and, more particularly, to an improved probe assembly for use with the battery impedance meter which minimizes excitation pick-up voltages.
Large battery systems are commonly used to provide backup power in case there is a failure of the commercial power grid. Typically, such a backup system includes a single string, or a plurality of parallel strings, of serially connected rechargeable battery cells and a charger connected to the commercial power grid for maintaining the charge on the battery cells. An inverter is coupled between the strings of battery cells and the load, which inverter is enabled upon the detection of a failure of the commercial power grid. In some applications, the inverter may be continuously operational to power the load with energy from the charger during the time that commercial power is available. Many of these battery backup systems, called “uninterruptible power supplies” (UPS), are configured such that the load is never aware of any failure of the commercial power grid because the battery system immediately supplies the necessary energy upon detecting a failure of the commercial power grid.
A typical installation of such an uninterruptible power supply is between the commercial power grid and a large computer system used by financial, communications, manufacturing and other commercial industries. If the battery system is taken “off-line” for any reason, the necessary protection against a power outage is lost for the time that the battery system is not connected plus the time for recharging, if a significant amount of charge has been removed during the off-line period of time. However, such battery backup systems must be monitored on a regular basis to insure that protection from commercial power grid failure is always available. Therefore, systems have been developed to perform such monitoring while the battery backup system remains on-line.
Impedance measurement is a method by which the condition of a battery cell may be assessed without taking the battery system off-line. Impedance measurements typically impose a current (hereinafter called the “loading current”) on the battery cell to be evaluated and measure the resulting voltage. Various commercially available test instruments function this way. Using Kelvin connections, these instruments impose a current on just the battery cell to be measured. After a measurement has been made, the operator moves the Kelvin clips to the next battery cell, reads the value, moves the clips to the next cell, and continues in this manner until all the battery cells have been measured. Therefore, the loading current flows almost entirely through the battery cell being measured, it being thought that the parallel paths (if they exist) are generally of so much higher impedance that any loading current flowing through them is of little or no consequence. However, FIG. 1 illustrates how conventional prior art measurement apparatus results in unavoidable errors.
FIG. 1 illustrates a typical two probe battery impedance meter 10 with a Kelvin connection to the battery cell 12, meaning that there are separate contacts to the battery for current drawn and voltage sensed. The meter 10 draws current (i) from the battery cell 12, symbolized by the current source 14. The drawn current is at a predetermined amplitude and frequency. While drawing the current, the meter 10 measures the voltage drop across the battery cell 12. The voltage measuring circuit in the meter is symbolized by the voltmeter 16. The ratio of voltage drop to current drawn is the internal impedance of the battery cell 12. Where the cables 18, 20 physically separate to the probes 22, 24 at the battery posts 26, 28, a one-turn coil is formed. The current passing through the wires going to the current source 14 produces a magnetic field in the formed coil. The magnetic field is made up of flux lines symbolized in FIG. 1 as circles with either a dot or a cross inside. The crosses signify that the flux lines are going into the page and the dots signify that the flux lines are coming out of the page. This magnetic field induces an excitation pick-up voltage in the wires, symbolized by vi. Faraday's law states that if a coil of N turns is placed in a region of changing flux (φ), a voltage (vi) will be induced across the coil according to equation (1):vi=N(dφ/dt).  (1)The meter 10 will therefore measure a voltage (v) according to equation (2):v=vi+vb,  (2)where vb is the actual voltage across the battery cell 12.
The term vi thus causes an error in measuring the battery impedance because battery impedance should only use the voltage drop across the battery (vb). It is difficult to compensate for vi because vi will change for different loop geometries. Since the spacing of the terminals 26, 28 differs from battery to battery, the loop geometries are different, and therefore vi is different and unpredictable.
It would therefore be desirable to be able to minimize the excitation pick-up voltage while taking battery cell impedance measurements.